[PATCH] kernel/groups.c: use bsearch library function

[PATCH] kernel/groups.c: use bsearch library function

[Thomas Meyer]

commit b7b2562f7252 ("kernel/groups.c: use sort library function")
introduced the sort library function.
also use the bsearch library function instead of open-coding the binary
search.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
---
 kernel/groups.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/kernel/groups.c b/kernel/groups.c
index daae2f2dc6d4f..69561a9cb4d39 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -2,6 +2,7 @@
 /*
  * Supplementary group IDs
  */
+#include <linux/bsearch.h>
 #include <linux/cred.h>
 #include <linux/export.h>
 #include <linux/slab.h>
@@ -96,22 +97,12 @@ EXPORT_SYMBOL(groups_sort);
 /* a simple bsearch */
 int groups_search(const struct group_info *group_info, kgid_t grp)
 {
-	unsigned int left, right;
-
 	if (!group_info)
 		return 0;
 
-	left = 0;
-	right = group_info->ngroups;
-	while (left < right) {
-		unsigned int mid = (left+right)/2;
-		if (gid_gt(grp, group_info->gid[mid]))
-			left = mid + 1;
-		else if (gid_lt(grp, group_info->gid[mid]))
-			right = mid;
-		else
-			return 1;
-	}
+	if (bsearch(&grp, group_info->gid, group_info->ngroups,
+		    sizeof(*group_info->gid), gid_cmp))
+		return 1;
 	return 0;
 }
 
-- 
2.21.0

Re: [PATCH] kernel/groups.c: use bsearch library function

[Rasmus Villemoes]

On 07/10/2019 21.26, Thomas Meyer wrote:
> commit b7b2562f7252 ("kernel/groups.c: use sort library function")
> introduced the sort library function.
> also use the bsearch library function instead of open-coding the binary
> search.

Yes, but please note the difference between sorting the group_info and
searching it: The former is done quite rarely - the setgroups syscall is
used roughly once per login-session.

But the searching of that structure is done more or less every time a
user accesses a file not owned by that user (e.g., any time a normal
user accesses anything in /usr) - at least if I'm reading
acl_permission_check() right.

So using a callback-based interface, especially in a post-spectre world,
may have a somewhat large performance impact.

Rasmus

Re: [PATCH] kernel/groups.c: use bsearch library function

[Thomas Meyer]

Rasmus Villemoes <linux@rasmusvillemoes.dk> writes:

> On 07/10/2019 21.26, Thomas Meyer wrote:
>> commit b7b2562f7252 ("kernel/groups.c: use sort library function")
>> introduced the sort library function.
>> also use the bsearch library function instead of open-coding the binary
>> search.

Hi,

> Yes, but please note the difference between sorting the group_info and
> searching it: The former is done quite rarely - the setgroups syscall is
> used roughly once per login-session.
>
> But the searching of that structure is done more or less every time a
> user accesses a file not owned by that user (e.g., any time a normal
> user accesses anything in /usr) - at least if I'm reading
> acl_permission_check() right.
>
> So using a callback-based interface, especially in a post-spectre world,
> may have a somewhat large performance impact.

okay, so the code is duplicated for performance reasons? nothing a
compiler can inline, I guess.

so what about a comment instead:

diff --git a/kernel/groups.c b/kernel/groups.c
index daae2f2dc6d4f..46b5d4cd53c2e 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -93,7 +93,7 @@ void groups_sort(struct group_info *group_info)
 }
 EXPORT_SYMBOL(groups_sort);
 
-/* a simple bsearch */
+/* duplicate code from lib/bsearch.c for performance reasons */
 int groups_search(const struct group_info *group_info, kgid_t grp)
 {
        unsigned int left, right;


Mfg
thomas